In many four-wheel drive vehicles one of the axles may be connected or disconnected from the power train to alternately provide a four-wheel drive mode or two-wheel drive mode of operation for the vehicle. In addition, an inter-axle differential may also be provided within the transfer case to facilitate vehicle operation in its four-wheel drive operating mode when the vehicle is driven on hard surface roads to prevent drive train components from being worn or damaged due to wind-up loads imposed by differences between front and rear axle speeds. In common parlance, a transfer case with an inter-axle differential provides "full-time" four-wheel drive, and without an inter-axle differential provides "part-time" four-wheel drive.
Generally, in a "full-time" four-wheel drive system, the vehicle operates in the four-wheel drive mode. However, it would be advantageous to extend the capability of a full-time system to provide a two-wheel drive mode when driving conditions do not require the added traction of four-wheel drive.
Prior patents disclose various apparatus and methods for changing a transfer case between a four-wheel drive operating state and a two-wheel drive operating state. For example, the U.S. Pat. of Pringle No. 2,754,695 discloses an air-pressure actuated shift mechanism associated with a transfer case. Likewise, the U.S. Pat. of Sampietro et al No. 3,221,574 discloses a method and apparatus for electromagnetically shifting a transfer case between a four-wheel drive operating state and a two-wheel drive operating state. The U.S. Pat. of Hammond No. 3,256,750 discloses a transfer case wherein a front drive shaft is electromagnetically uncoupled from the transfer case prior to manual shifting between a two-wheel drive and a four-wheel drive operating state of the transfer case to thereby relieve windup in the front drive shaft. In the U.S. Pat. of Kaiser No. 3,283,298 a transfer case is disclosed for allowing manual shifting in both high and low gear between a two-wheel drive operating state and a four-wheel drive operating state of the transfer case. The U.S. Pat. of Ordorica No. 3,295,625 discloses a transfer case which automatically shifts between a two-wheel drive operating state and a four-wheel drive operating state. The U.S. Pat. of Johnston, Jr. et al No. 3,354,741 discloses a transfer case which allows shifting between a two-wheel drive operating state and a four-wheel drive operating state when the transmission of the vehicle is in its neutral position.
Transfer cases which have full-time drive capability through the use of an inter-axle differential therein to permit operation in four-wheel drive on hard road surfaces are disclosed in many patents. For example, the U.S. Pat. of Jeffers No. 3,871,249 discloses an inter-axle differential of the lock-up type which includes inhibiting means for preventing lock-up in the event the speed of the input shaft exceeds a predetermined given value. Likewise, the U.S. Pat. of Pagdin et al No. 4,022,084 discloses a transfer case having an inter-axle differential associated with a control coupling and further including a chain and sprocket front drive transfer. In the U.S. Pat. of Dolan No. 3,848,691 a differential is provided in a transfer case wherein the differential may be locked up to provide a non-differentiating drive to the front and rear wheel sets by means of shiftable clutches which are manually operated by means of yokes.
Another transfer case of the type to which this invention relates is disclosed by the U.S. Pat. of Hart et al No. 3,788,166.